All versions of Firefox work on Windows XP. You need to check your hardware specifications.
* Firefox 4 - http://www.mozilla.com/en-US/firefox/system-requirements.html
* Firefox 3.6 - https://www.mozilla.com/en-US/firefox/3.6/system-requirements/
You can get Firefox 4 from http://www.mozilla.com/ and Firefox 3.6 from http://www.mozilla.com/en-US/firefox/all-older.html